Methods for manganese removal of cast iron

ABSTRACT

The present invention does not require a demanganese agent such as a sulfide or a combustible gas in the removal of manganese of cast iron. The method for removing manganese of cast iron according to the present invention is implemented by performing the removal of a manganese component by allowing a furnace to be in an oxygen atmosphere, and by blowing air into a molten cast iron in the furnace, while a carbon component in the molten cast iron is being maintained at an approximately constant amount. Alternatively, the method for removing manganese of cast iron according to the present invention is implemented by performing the removal of the manganese component by allowing the furnace to be in an oxygen atmosphere and by stirring the molten cast iron in the furnace, while the carbon component in the molten cast iron is being maintained at an approximately constant amount.

TECHNICAL FIELD

The present invention relates to a method for removing manganese in a feedstock having a high manganese content to be used for the production of cast iron members.

BACKGROUND ART

Cast iron castings have been used for vehicle components or machine components, approximately half of the amount of cast iron castings produced is for vehicles, and cast iron castings account for approximately 10% of the gross vehicle weight. The feedstock used for the production of cast iron castings utilizes steel scraps of automobile steel plates; the recent request for weight reduction has increased the manganese content in automobile steel plates; manganese is a pearlitization accelerating element, and accordingly causes a problem which is that the toughness is degraded and internal defects tend to be caused.

For the problem of the increase of the manganese content in the feedstock used for the production of cast iron members, the following proposals have been made. For example, Patent Literature 1 proposes a demanganese treatment method of cast iron in which manganese is removed from the molten metal by adding a sulfur-containing demanganese treatment agent to the molten cast iron containing manganese, to float manganese as manganese sulfide. In the removal of the manganese component, the MnS produced in the molten metal is floated, and removed into the slag on the surface of the molten metal. In order to promote the flotation removal, it is effective to stir the molten metal by, for example, blowing a gas from the porous plug in the bottom of the ladle containing the molten metal. In the case of the gas stirring, as the blowing-in gas, compressed air or compressed nitrogen gas is low in price and easy to use. In this regard, it is regarded as preferable to use an inert gas such as Ar in order to suppress the increase of the amount of oxygen or the amount of nitrogen in the molten metal.

Patent Literature 2 proposes a method for decreasing the manganese content in the production of a cast iron, wherein the method is a method for removing manganese in the molten cast iron by adding and mixing only sodium nitrate as an additive in the molten cast iron at a temperature of 1400 to 1500° C. In the method for removing manganese, when the temperature of the molten cast iron is lower than 1300° C., SiO₂ is produced in large amounts, and Si, a main component of the cast iron, is unpreferably depleted significantly. In addition, in the molten cast iron at 1300° C. or higher, MnS is regarded to be formed only in a region having an extremely high Mn or S content (%). According to this invention, it is possible to achieve a Mn removal rate of 70% or more even when the Mn content of the molten cast iron is 1.5% by mass or more. The Mn removal rate is described to be improved according to the amount of Na₂SO₄ added, when the amount of Na₂SO₄ added is approximately 10% by mass or less.

Patent Literature 3 proposes a method for producing a spheroidal graphite cast iron, wherein in the melting by using a rotary furnace by utilizing a heat source such as a natural gas, a liquefied petroleum gas or kerosene and pure oxygen, as a charge material raw metal, steel scraps and return scraps or only steel scraps are used, and an original molten metal obtained by performing a Mn-removal melting in an oxidative combustion period, and a molten metal preliminarily melted in another furnace and regulated in components are combined by a molten-metal combining device to produce the spheroidal graphite cast iron. In Example of the method for producing the spheroidal graphite cast iron, a material raw metal having mixing proportions of 60% of steel scraps and 40% of return scraps was charged from a material charge inlet into a rotary furnace, and 1.62% of silica sand and 0.30% of limestone were sparged as a forming agent on the material raw metal. It has been reported that the melting was performed while the volume ratio of pure oxygen and natural (CH₄) gas was being regulated in a range from 1.95 to 2.10.

Patent Literature 4 proposes a method for removing impurities in a molten cast iron, wherein the method is a method for removing impurities containing manganese (Mn) while the depletion of carbon (C) and silicon (Si) contained in a molten cast iron preliminary melted is being suppressed; under the condition that the temperature of the molten cast iron is maintained at 1250° C. or higher and lower than 1500° C., while the aforementioned molten metal and an acidic slag layer are being allowed to contact with each other, an excessive oxygen flame having a theoretical combustion ratio of fuel and oxygen (amount (volume) of oxygen×5/amount (volume) of fuel) of 1 to 1.5 is directly exposed to the surface of the molten cast iron to overheat the surface of the molten cast iron. The temperature of the molten cast iron at the time of feeding the molten cast iron is preferably lower than 1500° C., and more preferably 1250° C. or higher and lower than 1500° C. In such a temperature range, the Mn removal treatment after the feeding of the molten cast iron is made easy while the depletion of C or Si is being suppressed. The excessive oxygen flame is considered to be preferably a flame of a burner obtained by combusting LPG gas or LNG gas while feeding oxygen in an amount excessive than the amount of oxygen necessary for combustion. The excessive oxygen flame removes impurities while directly exposing the molten metal surface to the flame and allowing the rest of the molten metal surface to contact with the acidic slag, without increasing the temperature of the whole molten metal; thus, it is considered that the depletion of C or Si can be made small while the oxidation removal of Mn is proceeding.

CITATION LIST Patent Literature

-   [Patent Literature 1] Japanese Patent Laid-Open No. 2003-105420 -   [Patent Literature 2] Japanese Patent No. 4210603 -   [Patent Literature 3] Japanese Patent Laid-Open No. 7-268432 -   [Patent Literature 4] Japanese Patent Laid-Open No. 2011-153359

SUMMARY OF INVENTION Technical Problem

The method for removing manganese described in Patent Literature 1 or 2 uses a sulfide as a demanganese agent, requires the amount of the sulfide to be a few percent in terms of percent by mass, and suffers from a problem which is that a large amount of a sulfur-containing slag is produced. The method described in Patent Literature 2 gives a small increase of sulfur in the molten cast iron; however, the sulfur content is increased to 0.02 to 0.03%, and hence the method concerned suffers from a problem which is that desulfurization is required when the cast iron is used for spheroidal graphite cast iron members.

On the other hand, the method for removing manganese described in Patent Literature 3 or 4 is a method in which the manganese contained in the cast iron is oxidized and removed as slag, and accordingly has an advantage that the amount of the produced slag is small. In particular, the method described in Patent Literature 4, in contrast to the method described in Patent Literature 3, uses no slag forming flux, and accordingly the amount of the produced slag can be made small. However, the method for treating manganese described in Patent Literature 3 or 4 requires a combustible gas, and suffers from a problem that the method concerned is not preferable at a site of work in hot environment.

As a method for removing manganese of cast iron, in a society emphasizing environmental conservation, a method producing a large amount of slag and a sulfur-containing slag is not preferable. In addition, the method for removing manganese of cast iron is preferably a method requiring no combustible gas from the viewpoints of handleability and work efficiency. The treatment temperature in the removal of manganese of cast iron is preferably as low as possible from the viewpoint of energy saving.

In view of such conventional problems and requirements, an object of the present invention is to provide a method for removing manganese of cast iron, not requiring a demanganese agent such as a sulfide or a combustible gas in the removal of manganese of cast iron, being small in the amount of the produced slag, being high in the manganese removal efficiency, and allowing the work to be performed safely.

Solution to Problem

The method for removing manganese of cast iron according to the present invention is implemented by performing the removal of a manganese component by allowing a furnace to be in an oxygen atmosphere, and by blowing air into a molten cast iron in the furnace, while a carbon component in the molten cast iron is being maintained at an approximately constant amount.

Alternatively, the method for removing manganese of cast iron according to the present invention is implemented by performing the removal of the manganese component by allowing the furnace to be in an oxygen atmosphere, and by stirring the molten cast iron in the furnace, while the carbon component in the molten cast iron is being maintained at an approximately constant amount.

In the foregoing invention, the removal of the manganese component can be performed while the amount of oxygen fed into the furnace or/and the stirring speed of the molten cast iron in the furnace are being regulated, and the carbon component in the molten cast iron is being maintained at an approximately constant amount.

In the foregoing invention, the removal of the manganese component can also be performed while the ratio between the removal rate of the silicon component and the removal rate of the manganese component is being maintained at an approximately constant value, and the removal of the manganese component can be performed while the decrease of the silicon component is being suppressed.

In the foregoing invention, the removal of the manganese component can also be performed while the temperature of the molten cast iron is being maintained at an approximately constant value, and the temperature of the molten cast iron can be set at 1400° C. to 1200° C.

Advantageous Effects of Invention

The method for removing manganese of cast iron of the present invention provides a method for removing manganese, not requiring a demanganese agent such as a sulfide or a combustible gas, and being small in the amount of the produced slag, and can remove manganese in a high efficiency.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating the furnace used in the test A of the present invention.

FIG. 2 is a diagram illustrating the furnace used in the test B of the present invention.

FIG. 3 is a graph showing the test results of Example 1 in the test A.

FIG. 4 is a graph showing the Mn residual rates of the test A.

FIG. 5 is a graph showing the C residual rates of the test A.

FIG. 6 is a graph showing the Si residual rates of the test A.

FIG. 7 is a graph showing the temperatures of the molten cast irons of the test A.

FIG. 8 is a graph showing the Mn residual rates of the test B.

FIG. 9 is a graph showing the C residual rates of the test B.

FIG. 10 is a graph showing the Si residual rates of the test B.

FIG. 11 is a graph showing the temperatures of the molten cast irons of the test B.

FIG. 12 is a graph showing the relation between the stirring speed and the Mn removal rate of the test B.

FIG. 13 is a graph showing the Si/Mn decrease ratios.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the embodiments of the present invention are described. The method for removing manganese according to the present invention is a method for removing manganese of cast iron by allowing a furnace to be in an oxygen atmosphere, and by blowing air into the molten cast iron to remove the manganese component contained in the molten cast iron, wherein the method concerned is implemented by performing the removal of the manganese component while the carbon component in the molten cast iron is being maintained at an approximately constant amount. In other words, the present method for removing manganese performs the manganese removal treatment by blowing air into the molten cast iron in the furnace allowed to be in an oxygen atmosphere, and accordingly belongs to an oxygen treatment method. In addition, the manganese removal treatment is implemented by performing the removal of the manganese component while the carbon component is maintained at a constant amount.

In the present invention, for the feeding of oxygen, it is important to allow the furnace to be in an oxygen atmosphere. Although oxygen can be fed by bubbling with air, it is preferable to feed oxygen by providing a dedicated unit capable of feeding oxygen onto the surface of the molten cast iron in the furnace. In the present invention, the method for feeding oxygen and the amount of oxygen fed affects the effect of the manganese removal treatment. In the present invention, by regulating the amount of oxygen fed, the removal rate of the manganese component in the molten cast iron, the temperature of the molten metal or the like can be regulated.

The blowing of air into the molten cast iron expands the reaction interface between the oxygen in the air blown into the molten cast iron by the fluidization/stirring of the molten cast iron and the oxygen fed into the furnace, and thus promotes the oxidation of manganese. Accordingly, an inert gas is not preferable. A mixed gas of air and oxygen is not preferable because such a mixed gas causes oxidation/combustion. The degree of the blowing of air into the molten cast iron is not required to have an amount or strength to scatter the molten metal or the slag. In other words, the blowing of air into the molten cast iron is not required to be an intense bubbling.

The temperature of the molten metal in performing the manganese removal treatment is favorably 1400° C. or lower for the purpose of suppressing the consumption of carbon in the molten cast iron, and can be regulated to be 1350° C. to 1175° C. The temperature of the molten cast iron is preferably as low as possible from the viewpoint of energy saving; however, in consideration of the energy saving inclusive of the successive steps, the temperature of the molten cast iron is preferably 1350° C. to 1200° C. Specifically, this is because the molten cast iron having been subjected to the removal of Mn is increased in temperature to 1400 to 1550° C., and then used for casting or the like.

The furnace for treating the molten cast iron in the present invention may be a tool having no heating unit by itself such as a ladle, or alternatively, may be a furnace being capable of allowing the furnace to be in an oxygen atmosphere and having an air feeding unit capable of blowing air into the molten cast iron. For example, such a ladle as shown in FIG. 1 can be used. In FIG. 1, a furnace 10 has a furnace body 11, a furnace lid 12, an air feeding unit 15 capable of blowing air into a molten cast iron 20, and an oxygen feeding unit 16 capable of allowing the furnace 10 to be in an oxygen atmosphere. In addition, the furnace 10 also has an operation opening 12 a for performing the taking out of a sample, and an exhaust outlet 12 b for discharging the gas generated during the treatment operation.

As described above, in the method for removing manganese of cast iron according to the present invention, by blowing air into the molten cast iron in an oxygen atmosphere to subject the molten cast iron to fluidization/stirring, the area of the reaction interface between the molten cast iron and oxygen is expanded to promote the oxidation of Mn. Such a method of subjecting the molten cast iron to fluidization/stirring may be a method of directly stirring the molten cast iron. Specifically, the furnace is allowed to be in an oxygen atmosphere, then the molten cast iron in the furnace is stirred, and thus the removal of the manganese component can be performed while the carbon component in the molten cast iron is being maintained at an approximately constant amount. Such a method of directly stirring the molten cast iron has an advantage of being relatively simple to control.

The method of directly stirring the molten cast iron is small in the temperature decrease of the molten cast iron, and allows the manganese component to be removed at an approximately constant temperature while the carbon component in the molten cast iron is being maintained at an approximately constant amount. In addition, the method of directly stirring the molten cast iron can regulate the removal rate of the manganese component in the molten cast iron by regulating the stirring speed or the stirring force in the stirring of the molten cast iron. Moreover, by regulating the stirring speed or the stirring force in the stirring of the molten cast iron, the molten cast iron can be maintained at an approximately constant temperature, or can be increased in temperature.

As described above, the present invention can perform an efficient removal of the manganese component in the molten cast iron, by allowing the furnace to be in an oxygen atmosphere, and by regulating the amount of oxygen fed or/and the stirring speed or the stirring force in the stirring of the molten cast iron. The furnace 10 shown in FIG. 2 can implement the foregoing method of performing the removal of the manganese component in the molten cast iron by allowing the furnace to be in an oxygen atmosphere and by stirring the molten cast iron in the furnace. The furnace 10 has the furnace body 11, the furnace lid 12, a stirring unit 17 for stirring the molten cast iron 20, and the oxygen feeding unit 16 capable of allowing the furnace 10 to be in an oxygen atmosphere. In addition, the furnace 10 has the operation opening 12 a for performing the taking out of a sample, and the exhaust outlet 12 b for discharging the gas generated during the treatment operation. The furnace 10 of the present example mechanically stirs the molten cast iron 20 by the stirring unit 17 having a driving source such as a motor; however, there may be adopted a furnace having a stirring unit capable of stirring the molten cast iron on the basis of an electromagnetic method involving a high frequency wave or the like.

EXAMPLE 1

By using the furnace shown in FIG. 1, a manganese removal test (test A) of a cast iron was performed. During the present test, the measurement of the components of the molten cast iron was performed for the specimens sampled at appropriate times from the furnace by using an emission spectrophotometer (PDA-7020, manufactured by Shimadzu Corp.). The temperature measurement of the molten cast iron was performed by using an immersion type thermocouple. The amount of the molten cast iron poured was set to be 500 kg or 300 kg (only in Example 2). The oxygen feeding unit for feeding oxygen used a burner capable of feeding only oxygen or a mixed gas of oxygen and propane gas (LPG), or a sonic nozzle capable of feeding oxygen at a supersonic speed. When only oxygen was fed with a burner, the oxygen flow rate was 5 Nm³/h. When oxygen was fed with a sonic nozzle, oxygen flow rate was 3 Nm³/h. The inner diameter of the gas feed opening of the burner was approximately 15 mm, and the inner diameter of the gas feed opening of the sonic nozzle was approximately 2 mm.

The test conditions of the test A are shown in Table 1. In Table 1, the treatment time means the elapsed time after the start of the manganese removal test in which the molten cast iron melted in a cast iron melting furnace was poured into a preheated furnace. In Example 1, the blowing of air into the molten cast iron was performed at a flow rate of 200 L/min, and the feeding of oxygen was first performed with a burner for 15 minutes, and then successively performed with a sonic nozzle to an elapsed time of 34 minutes. In Example 2, only the blowing of air (400 L/min) was performed. In Example 3, the blowing of air (200 L/min) and the feeding of oxygen were performed. In Comparative Example 1, a mixed gas of oxygen and LPG was fed with a burner, and the blowing of air was first performed at a flow rate of 200 L/min for 21 minutes and then successively performed at an increased flow rate of 400 L/min to an elapsed time of 41 minutes. In Comparative Example 2, the blowing of air (200 L/min) and the feeding of oxygen were performed. In addition, for 15 minutes from the start of the test, 10 to 20 kg of charcoal was intermittently input in the furnace. When charcoal was input, active flame was observed from the sample input opening. It is to be noted that in Comparative Example 1, the mixed gas of oxygen and LPG was an excessive oxygen gas in relation to the theoretical combustion gas. When a sonic nozzle is used, oxygen can be fed at an ultrafast speed (equal to or faster than sonic speed). When the blowing of air was performed at a flow rate of 400 L/min, fierce bubbling occurred, but when the blowing of air was performed at a flow rate of 200 L/min, such fierce bubbling did not occur.

TABLE 1 COMPARATIVE COMPARATIVE EXAMPLE 1 EXAMPLE 2 EXAMPLE 3 EXAMPLE 1 EXAMPLE 2 TREATMENT AIR AIR AIR AIR AIR TIME OXYGEN L/ OXYGEN L/ OXYGEN L/ OXYGEN LPG L/ OXYGEN L/ min Nm³/h min Nm³/h min Nm³/h min Nm³/h Nm³/h min Nm³/h min 0 5 200 0 400 5 200 18 3 200 0 200 3 ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ 10 ↓ ↓ ↓ ↓ ↓ ↓ 24 4 ↓ ↓ ↓ 15 ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ 17 3 ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ 21 ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ 26 ↓ ↓ ↓ ↓ ↓ ↓ 400 ↓ ↓ 34 ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ 35 ↓ ↓ ↓ ↓ ↓ ↓ ↓ 40 ↓ ↓ ↓ ↓ ↓ 41 ↓ ↓ ↓

The results of the test A of Example 1 are shown in Table 2. In Table 2, the contents of the components are given in percent by mass, the residual components other than the components shown in Table 2 are iron and inevitable impurities. For the components other than foregoing manganese (Mn), carbon (C) and silicon (Si), according to Table 2, the content of titanium (Ti) was 0.017% at the beginning, was reduced to 0.008% after a treatment time of 34 minutes, and thus was reduced nearly by half. The content of aluminum (Al) was reduced by approximately 20% after a treatment time of 34 minutes, and the contents of chromium (Cr), boron (B) and zinc (Zn) are seen to be reduced to some extents.

TABLE 2 TEMPERATURE TREATMENT OF TIME MOLTEN min METAL° C. Mn % C % Si % Cr % Ti % Al % B % Zn % 0 1257 0.896 3.82 2.40 0.07 0.017 0.019 0.0008 0.003 5 1218 0.837 3.88 2.36 0.07 0.017 0.018 0.0008 0.004 10 1197 0.783 3.88 2.31 0.07 0.016 0.017 0.0012 0.004 15 1181 0.700 3.83 2.21 0.07 0.013 0.016 0.0006 0.003 20 1152 0.631 3.90 2.17 0.06 0.010 0.015 0.0005 0.003 29 1159 0.584 3.91 2.09 0.06 0.009 0.015 0.0004 0.002 34 1181 0.538 3.87 2.02 0.06 0.008 0.015 0.0007 0.002

The results of Example 1 are shown in FIG. 3. In FIG. 3, the abscissa represents the treatment time, and the ordinate represents the residual rate of Mn, Si or C, or the temperature of the molten metal. The residual rate means the rate of the residual content in relation to the initial content of Mn, C or Si. According to FIG. 3, the residual rate of C falls within a range from 1.00 to 1.02, and thus the content of C is maintained at an approximately constant value falling within a variation range of 2% of the initial content thereof. In contrast, the Mn residual rate curve is a steep downward curve, and the residual rate of Mn after a treatment time of 34 minutes is 0.6 (60% of the initial content), showing that the amount of Mn is rapidly decreased. On the other hand, the Si residual rate curve declines slowly, and the residual rate of Si after a treatment time of 34 minutes is 0.84 (84% of the initial content). In other words, in the present Example, Mn is removed at a speed faster by a factor of 2 or more as compared with Si. The temperature of the molten metal is gradually decreased from the initial temperature of 1257° C. to 1152° C., then gradually increased after an elapsed treatment time of 20 minutes, and reached 1181° C. after an elapsed treatment time of 34 minutes. This is understood that the effect of the sonic nozzle was manifested.

The graphs in FIGS. 4 to 7 show the relations of the Mn residual rate, the C residual rate, the Si residual rate and the temperature of the molten metal with the treatment time in the present test A. In FIGS. 4 to 6, the abscissa represents the treatment time, and the ordinate represents the residual rate of Mn, C or Si. In FIG. 7, the abscissa represents the treatment time, and the ordinate represents the temperature of the molten metal. In the Mn residual rate curve or the Si residual rate curve, the gradient or the removal rate means the ratio of the amount of a component removed per unit treatment time to the initial content ((residual rate a−residual rate b)/(treatment time b−treatment time a)).

<Blowing of Combustion Gas>

Comparative Example 1 (symbol: □) is a case where a manganese removal test was performed in an excessive oxygen furnace, by blowing LPG combustion gas to the surface of the molten cast iron. Comparative Example 1 is an example of the case where the amount of oxygen and the amount of LPG gas were increased after an elapsed treatment time of 10 minutes, and the blowing of air into the molten cast iron was also increased from 200 L/min to 400 L/min after an elapsed treatment time of 26 minutes. According to FIG. 4, the gradient of the Mn residual rate curve of Comparative Example 1 is smaller than the gradients of the Mn residual rate curves of Examples 1 to 3, and the Mn removal rate of Comparative Example 1 is approximately 80% of the Mn removal rates of Examples 1 to 3. According to FIG. 5, the removal of C is approximately 4% at an elapsed treatment time of 40 minutes even under the condition of an increased amount of air, showing that the removal of C (consumption of C) is suppressed. The gradient of the Si residual rate curve shown in FIG. 6 is the smallest until an elapsed treatment time of 20 minutes, and the removal of Si is also suppressed.

The effects of the increase of the amount of oxygen and the increase of the amount of LPG gas are not manifested in the Mn residual rate curves (FIG. 4), and also little manifested in the C residual rate curves (FIG. 5) and in the Si residual rate curves (FIG. 6). However, the temperature of the molten metal is increased after an elapsed treatment time of 10 minutes, well corresponding to the increase of the amount of oxygen and the increase of the amount of LPG gas. On the other hand, the effect of the increase of the amount of air is manifested in the Mn residual rate curves, and is clearly manifested in the C residual rate curves and the Si residual rate curves.

<Furnace Atmosphere>

Comparative Example 2 (symbol: ◯) is a case where the feeding of oxygen to the molten cast iron and the blowing of air into the molten cast iron were the same as in Example 1 or 3, but the manganese removal test was performed under the condition that charcoal was input in the furnace to alter the environment in the furnace from the case of Example 1 or 3. As shown in FIG. 4, the gradient of the Mn residual rate curve is most gentle, and the Mn removal rate of Comparative Example 2 is approximately 40% of the Mn removal rate of Example 1 or 3. According to the C residual rate curves shown in FIG. 5, the carburization effect due to charcoal is observed, but the amount of the residual C falls within a variation range of 4% of the initial content. According to the Si residual rate curve shown in FIG. 6, the Si residual rate is highest, and even after an elapsed treatment time of 30 minutes, the residual rate is 0.93. In other words, it is understood that the furnace offers an environment unlikely to allow oxidation to occur, and offers an environment suppressing the oxidation of Si. According to the temperature curves of the molten metals shown in FIG. 7, the increase of the temperature of the molten metal due to the combustion of charcoal is not observed, the temperature curve of the molten metal of Comparative Example 2 overlaps with the temperature curve of the molten metal of Example 3. It is to be noted that the straight line of indicator a shown in FIG. 4 and the straight line of indicator a shown in FIG. 6 are the straight lines having the same gradient. In other words, the Mn removal rate of Comparative Example 2 shown in FIG. 4 is almost the same as the Si removal rates of Examples 1 to 3 shown in FIG. 6.

EXAMPLE 2

By using the furnace shown in FIG. 2, a manganese removal test (test B) of a cast iron was performed. The test was performed by variously changing the oxygen feeding conditions or the stirring conditions _(o)f the molten cast iron as shown in Table 3. In Examples 4 to 6, the blowing of air at 200 L/min as well as the feeding of oxygen was performed. The blowing of air was performed by the same method as in Example 1 or 3 of the test A. In Examples 7 to 9, oxygen was fed at 20 Nm³/h from the start to the end of the test, and stirring speed of the stirring unit 17 was variously changed. For example, in Example 7, the stirring speed of the stirring unit 17 was 200 rpm from the start to the end of the test; in Example 8, the test was started at 100 rpm and the stirring speed was increased to 200 rpm after an elapsed treatment time of 11 minutes. In the present test, the measurement of the components of the molten cast iron or the measurement of the temperature of the molten cast iron was performed by using the same emission spectrophotometer as in the case of the test A or by using the same immersion type thermocouple as in the case of the test A, respectively. The amount of the molten cast iron poured into the furnace was set to be 500 kg.

TABLE 3 TREATMENT TIME min 0 7 11 15 16 29 OXYGEN EXAMPLE # FEEDING RATE Nm³/h REMARKS EXAMPLE 4 50 20 ← 10

AIR: 200 L/min EXAMPLE 5 15 ← 20 ← 25

AIR: 200 L/min EXAMPLE 6 25 15 ← ← ← 25

AIR: 200 L/min EXAMPLE 7 20

STIRRING: 200 rpm EXAMPLE 8 20

STIRRING: 100 to 200 (11 min)rpm EXAMPLE 9 20

STIRRING: 150 to 250(20 min)rpm

The results of the test B are shown in FIGS. 8 to 11. FIG. 8 shows the Mn residual rates, FIG. 9 shows the C residual rates, and FIG. 10 shows the Si residual rates; in each of these figures, the abscissa represents the treatment time, and the ordinate represents the residual rate of Mn, C or Si. In FIG. 11, the abscissa represents the treatment time, and the ordinate represents the temperature of the molten metal. The Mn residual rates shown in FIG. 8 include the results of the test B as well as the results of the test A of Examples 1 and 3. The straight line of indicator a and the straight line of indicator b in FIG. 8 have the same gradients as the gradients of the straight line of indicator a and the straight line of indicator b shown in FIG. 6 or FIG. 10. The gradients of the straight line of indicator a, the straight line of indicator b, and the straight line of indicator c are 1:3.2:6.1 with reference to the gradient of the straight line of indicator a.

According to FIG. 8, the Mn residual rates decrease approximately along the straight line of indicator b or the straight line of indicator c from the start of the treatment to an elapsed treatment time of 10 to 20 minutes. The Mn residual rates of Example 1, Example 3, Example 5 and Example 8 decrease along the straight line of indicator b. The Mn residual rates of Example 4, Example 6, Example 7 and Example 9 decrease along the straight line of indicator c. The Mn residual rate curves of Example 5 and Example 8 flex downward at an elapsed treatment time of 11 minutes, and have forms different from the forms of the other Mn residual rate curves.

According to FIG. 9, the C residual rates fall approximately within a range from 1.02 to 0.96, and are approximately constant values. In other words, the removal of C (decarburization) is suppressed. According to FIG. 10, the Si residual rate of each of Examples decreases approximately along the straight line of indicator b. The Si residual rate of Example 5 decreases at first along the straight line of indicator a, decreases rapidly after an elapsed treatment time of 16 minutes, and decreases after an elapsed treatment time of 25 minutes along the straight line of indicator b. In the case of Example 9, the removal of Si most proceeds. FIG. 11 shows that the treatment proceeds at a molten metal temperature of 1410 to 1270° C., the temperature curves of the molten metals are generally raised midway of the treatment, and the temperatures of the molten metals are increased.

<Feeding of Oxygen>

Examples 1, 3 and 4 to 6 shown in FIG. 8 are the same as each other with respect to the blowing of air at 200 L/min, but are different from each other with respect to the amount of oxygen fed or the feeding manner of oxygen. From the observation of these Mn residual rate curves, it can be seen that the amount of oxygen fed can regulate the Mn removal rate. Specifically, according to the Mn residual rate curve of Example 4, the gradient is the largest until an elapsed treatment time of 7 minutes in which a large amount of oxygen (50 Nm³/h) was fed, and 30% of the initial Mn content is removed at an elapsed treatment time of 12 minutes. It is understood that a sufficient feeding of oxygen at the beginning was effective. In the case of Example 5, due to the increase of the feeding of oxygen from the initial flow rate of 15 Nm³/h to the flow rate of 20 Nm³/h after an elapsed treatment time of 11 minutes, the Mn residual rate curve flexes after an elapsed treatment time of 11 minutes, and thus a promotion of the removal of Mn is observed.

The amounts of residual C of Examples 1, 3 and 4 to 6 fall within a range from 1.02 to 0.97 according to FIGS. 5 and 9, and are approximately constant values. The C residual rate is little affected by the amount of oxygen fed and the feeding manner of oxygen. On the other hand, FIGS. 6 and 10 show that the Si residual rate is affected by the amount of oxygen fed and the feeding manner of oxygen. Generally, when the amount of oxygen fed is small, the Si residual rate decreases along the straight line of indicator a, and when the amount of oxygen fed is large, the Si residual rate decreases along the straight line of indicator b. The effect of the variation of the amount of oxygen fed is well manifested in the case of Example 5. Specifically, the Si residual rate decreases along the straight line of indicator a when the feeding flow rate of oxygen is 15 Nm³/h, and rapidly decreases when the feeding flow rate of oxygen is increased to 20 Nm³/h. In addition, the Si residual rate decreases along the straight line of indicator b after an elapsed treatment time of 25 minutes. On the other hand, in Example 4 where a large amount of oxygen (50 Nm³/h) was fed at the beginning, and the amount of oxygen fed was drastically decreased at an elapsed treatment time of 7 minutes (20 Nm³/h) and at an elapsed treatment time of 15 minutes (10 Nm³/h), the effect of such a variation of the feeding flow rate of oxygen is little manifested in the Si residual rate curves, the C residual rate curves and the temperature curves of the molten metals (FIGS. 9 to 11).

From a comparison of FIG. 7 with FIG. 11, the temperature of the molten metal of Example 1 or 3 decreases by approximately 100° C. during the treatment. In contrast, the decreases of the temperatures of the molten metals of Examples 4 to 6 are small, and the decrease of the temperature in Example 4 is 55° C. at maximum. It is understood that when the amount of oxygen fed is equal to or larger than a predetermined amount (for example, 15 Nm³/h), the decrease of the temperature of the molten metal due to the blowing of air into the molten metal can be suppressed. As can be seen from FIG. 11, the increase of the amount of oxygen tends to increase the temperature of the molten metal. In other words, by regulating the amount of oxygen fed, the regulation of the temperature of the molten metal can be performed.

<Stirring of Molten Metal>

As shown in Table 3, Examples 7, 8 and 9 are the tests in which the oxygen feeding was performed at 20 Nm³/h, and the stirring conditions of the molten cast iron were varied. Example 7 is the case where the test was performed at a stirring speed of 200 rpm. According to Example 7, Mn can be removed efficiently (FIG. 8), the decarburization is low (FIG. 9), the decrease of the temperature of the molten metal is as small as approximately 20° C., and the Mn removal can be performed at an approximately constant temperature of the molten metal (FIG. 11). In the case of Example 8, due to the increase of the stirring speed from the initial speed of 100 rpm to 200 rpm after an elapsed treatment time of 11 minutes, the Mn residual rate decreases initially along the straight line of indicator b, but rapidly decreases and decreases along the straight line of indicator c after an elapsed treatment time of 18 minutes. In other words, it is shown that by regulating the stirring speed of the molten metal, the degree of the Mn removal can be regulated. According to FIG. 8, the Mn residual rates of Examples 7 to 9 decrease linearly up to an elapsed treatment time of 10 minutes. The relation between the gradient (Mn removal rate) of the Mn residual rate curve and the stirring speed is shown in FIG. 12. According to FIG. 12, a certain relation is observed between the Mn removal rate and the stirring speed. Accordingly, the degree of the removal of Mn can be regulated by regulating the stirring speed of the molten metal on the basis of FIG. 12.

According to FIG. 11, in Example 8, the temperature of the molten metal increases after an elapsed treatment time of 15 minutes, and this well corresponds to the increase of the stirring speed from 100 rpm to 200 rpm after an elapsed treatment time of 11 minutes. Example 9 is an example where the treatment was started at a stirring speed of 150 rpm and the stirring speed was increased to 250 rpm after an elapsed treatment time of 20 minutes, the temperature of the molten metal increases after an elapsed treatment time of 20 minutes, and the effect of the alteration of the stirring speed is clearly manifested. In the case of Example 9, as shown in FIGS. 8 to 10, the effect of the alteration of the stirring speed is little manifested in the Mn residual rate, the C residual rate and the Si residual rate.

<Removal of Mn and Removal of Si>

According to FIG. 8, the Mn residual rates decrease along the straight line of indicator b and the straight line of indicator c. According to FIGS. 6 and 10, the Si residual rates decrease along the straight line of indicator b and the straight line of indicator c. The ratio between the gradient of the straight line of indicator b and the gradient of the straight line of indicator c is 32/61 and hence approximately 1/2. In other words, the decrease rate ratio is approximately 1/2.

FIG. 13 is a graph showing the relation between the Si/Mn decrease ratio ((1.00−Si residual rate a)/(1.00−Mn residual rate a)) at an elapsed treatment time of a and the treatment time. According to FIG. 13, the Si/Mn decrease ratio of Example 7 is approximately 0.5 and approximately constant. In other words, the decrease rate of Si is 1/2 of the decrease rate of Mn. According to FIG. 13, the Si/Mn decrease ratio of Example 9 is approximately 0.8 and approximately constant, but the Si/Mn decrease rate of Example 4 vibrates within a range from 0.1 to 0.7. The Si/Mn decrease ratio of Example 6 increases approximately linearly within a range from 0.62 to 0.9. The Si/Mn decrease ratio of Example 8 increases approximately linearly within a range from 0.01 to 0.5, and the removal (consumption) of Si is suppressed.

In the method for removing manganese of molten cast iron, preferably the degree of the removal of Si can be predicted in relation to the removal of Mn. From such a viewpoint, the method of Example 7 or 9 is preferable.

<Cr, Ti, Al, B and Zn>

The present invention can remove the metal components such as Cr, Ti, Al, B and Zn. Table 4 shows the results obtained in Example 5, and Table 5 shows the results obtained in Example 7. In each of these tables, the time means the treatment time, and the temperature means the temperature of the molten metal. The contents of the respective components are given in percent by mass. From a comparison between Table 4 and Table 5, B can be efficiently removed in both of the case of Example 5 and the case of Example 7. However, the removal of Zn is difficult in the case of Example 5 based on the blowing of air, and the removal of Cr and Al is difficult in the case of Example 7 based on the stirring of the molten metal. According to Example 5, a treatment of approximately 30 minutes can remove 40 to 50% of Cr, Ti or Al. According to Example 7, a treatment of 15 minutes can remove 50 to 60% of Ti or Zn.

TABLE 4 TIME min TEMPERATURE ° C. Cr % Ti % Al % B % Zn % 0 — 0.05 0.019 0.027 0.0007 0.003 4 — 0.05 0.019 0.023 0.0007 0.003 11 1343 0.05 0.018 0.023 0.0005 0.002 16 1328 0.04 0.017 0.021 0.0004 0.002 21 1350 0.04 0.014 0.019 0.0002 0.002 26 1360 0.04 0.011 0.017 0 0.002 31 1377 0.03 0.009 0.016 0 0.002

TABLE 5 TIME min TEMPERATURE ° C. Cr % Ti % Al % B % Zn % 0 1336 0.05 0.008 0.015 0.0002 0.0019 5 1329 0.05 0.007 0.014 0.0001 0.0014 10 1326 0.05 0.005 0.014 0 0.0009 15 1315 0.05 0.004 0.014 0 0.0007

REFERENCE SIGNS LIST

-   10 furnace -   11 furnace body -   12 furnace lid -   15 air feeding unit -   16 oxygen feeding unit -   20 molten cast iron 

1. A method for removing manganese of cast iron, wherein the removal of a manganese component is performed by allowing a furnace to be in an oxygen atmosphere by introducing oxygen gas into the furnace, and by blowing air into a molten cast iron in the furnace, while a carbon component in the molten cast iron is being maintained at an approximately constant amount.
 2. A method for removing manganese of cast iron, wherein the removal of a manganese component is performed by allowing a furnace to be in an oxygen atmosphere by introducing oxygen gas into the furnace, and by stirring a molten cast iron in the furnace, while a carbon component in the molten cast iron is being maintained at an approximately constant amount.
 3. The method for removing manganese of cast iron according to claim 2, wherein the removal of the manganese component is performed while the amount of oxygen fed into the furnace or/and the stirring speed of the molten cast iron in the furnace are being regulated.
 4. The method for removing manganese of cast iron according to claim 1, wherein the removal of manganese is performed while the ratio between the removal rate of a silicon component and the removal rate of the manganese component is being maintained at an approximately constant value.
 5. The method for removing manganese of cast iron according to claim 1, wherein the removal of manganese is performed while the decrease of a silicon component is being suppressed.
 6. The method for removing manganese of cast iron according to claim 1, wherein the removal of manganese is performed while the temperature of the molten cast iron is being maintained at an approximately constant temperature.
 7. The method for removing manganese of cast iron according to claim 1, wherein the temperature of the molten cast iron is 1400° C. to 1200° C.
 8. A method for removing a metal component of cast iron, wherein the removal of a manganese component as well as the removal of the metal component such as chromium, titanium, aluminum, boron or zinc is performed by allowing a furnace to be in an oxygen atmosphere by introducing oxygen gas into the furnace, and by blowing air into a molten cast iron in the furnace, or by allowing the furnace to be in an oxygen atmosphere, and by stirring the molten cast iron in the furnace, while a carbon component in the molten cast iron is being maintained at an approximately constant amount.
 9. The method for removing manganese of cast iron according to claim 2, wherein the removal of manganese is performed while the ratio between the removal rate of a silicon component and the removal rate of the manganese component is being maintained at an approximately constant value.
 10. The method for removing manganese of cast iron according to claim 2, wherein the removal of manganese is performed while the decrease of a silicon component is being suppressed.
 11. The method for removing manganese of cast iron according to claim 2, wherein the removal of manganese is performed while the temperature of the molten cast iron is being maintained at an approximately constant temperature.
 12. The method for removing manganese of cast iron according to claim 2, wherein the temperature of the molten cast iron is 1400° C. to 1200° C. 